c / asm / stack and pointers

[Lynux-Penguin]

When you declare a variable i.e.

int k;

how exactly does it create it in the stack...
or better yet, explain this...

int main()
{
    int k=5;
    return 0;
}

becomes:

.text
        .align 4
.globl main
        .type    main,@function
main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $4, %esp
        movl    $5, -4(%ebp)
        movl    $0, %eax
        leave
        ret

my explanation (please correct me)

pushl %ebp - pushes the frame pointer onto the stack to be popped later
movl %esp, %ebp - moves the current stack pointer to become the new frame pointer
subl $4, %esp - subtract 4 bytes (one word) from the stack\
    pointer, i.e. move the pointer up four to make room for the var.
movl $5, -4(%ebp) - assign the value 5 to the address of ebp offsetted -4 (4 up)
movl $0, %eax - the return value
leave - you know
ret - return %eax;

correct me if I'm wrong but if i made another function called foo() could I use foo to get back at the -4(%ebp) and how? without parameters...
(btw: I think this is ILLEGAL C because it is compiler specific on how the stack is arranged but just for the point of learning, assume its on gcc and a linux system)
using pointers...

and another question
is there a var in C that stores the value of EBP and ESP etc. without using inline asm? To my knowledge there is not but if there is, i would like to learn of it...

Thanks for any input
-LC

[Fordy]

Originally posted by Lynux-Penguin
my explanation (please correct me)

pushl %ebp - pushes the frame pointer onto the stack to be popped later
movl %esp, %ebp - moves the current stack pointer to become the new frame pointer
subl $4, %esp - subtract 4 bytes (one word) from the stack\
    pointer, i.e. move the pointer up four to make room for the var.
movl $5, -4(%ebp) - assign the value 5 to the address of ebp offsetted -4 (4 up)
movl $0, %eax - the return value
leave - you know
ret - return %eax;

For reference, the leave instruction will set ESP to the value in EBP, then it will pop into EBP - the opposite of the first few instructions

Originally posted by Lynux-Penguin
correct me if I'm wrong but if i made another function called foo() could I use foo to get back at the -4(%ebp) and how? without parameters...
(btw: I think this is ILLEGAL C because it is compiler specific on how the stack is arranged but just for the point of learning, assume its on gcc and a linux system)
using pointers...

and another question
is there a var in C that stores the value of EBP and ESP etc. without using inline asm? To my knowledge there is not but if there is, i would like to learn of it...

Thanks for any input
-LC

It is possible to traverse the stack to find varibles from calling functions (assuming those variable are normal stack variables - and not static...etc). You cant do it in straight C unless (using your example) main calls foo with a pointer to a variable declared in main..

[Lynux-Penguin]

ok yeah, i thought so, thanks a lot

-LC

[VirtualAce]

Man you guys are killing me with this AT&T crap.

.text
        .align 4

.global main
        .type    main,@function

main:
        push ebp               ;pushl   %ebp
        mov  esp,ebp         ;movl    %esp, %ebp
        sub   esp,4             ;subl    $4, %esp
        mov   [ebp-4],5      ;movl    $5, -4(%ebp)
        mov   eax,0            ;movl    $0, %eax
        leave
        ret

Much better.

[Lynux-Penguin]

what ever gets the job done.
I like the AT&T Version MORE now, simply because I use it a lot more than the other kinds.

-LC

[Hammer]

Since when has this become an asm forum? Please try to stay on topic

[VirtualAce]

Since when has this become an asm forum? Please try to stay on topic

Thanks for making us feel right at home. It wouldn't be the same if someone didn't whine about us using asm, even if yours was completely sarcastic.

I take it you read the thread where I got blasted for using assembly. Told off is more like it.

[golfinguy4]

Are you talking about the thread where the power-tripping immature brat made a fool of himself? Ah, we are talking about the same thread.

[Lynux-Penguin]

> Please try to stay on topic

Well, if you see my original thread, there was C. It just slowly got mutated into ASM because the question was easier to understand if we ripped it apart into ASM.

However, gotcha

-LC

[VirtualAce]

Are you talking about the thread where the power-tripping immature brat made a fool of himself? Ah, we are talking about the same thread.

That would be the one.